It has been the practice in the past to gauge the internal neck diameter of containers by the insertion of a plug of specific diameter into the neck. In those instances where the plug would not pass into and through the neck of the container, the container would be termed as having a "choked finish" or neck. One example of such a gauge is that of U.S. Pat. No. 2,596,342 in which a plurality of inspection or plug gauging heads are carried by a unit positioned above a moving conveyor wherein the conveyor carries the bottles in series beneath the plurality of gauging units. It was necessary to have an inspection device composed of a plurality of gauging units in order to inspect containers at the rate in which they were being produced by forming machines. It was expected that one multiple head plug gauge would be positioned at the cold end of the lehr that is being used to anneal the containers that are made by machines. In the past, there would be perhaps a flow of 180 bottles a minute being gauged. The present-day bottle lines now are running at even greater speeds and volume, such that a gauge capable of handling up to 400 or so bottles per minute would be highly desirable.
In addition to the known plug gaugers in the prior art, there is a need for a high speed apparatus which will also determine whether or not the neck or finish portion of the container has been completely formed. As has occurred during the production of glassware, sometimes the finish is unfilled, meaning that during the formation of the neck of the container, it was not completely formed in the neck ring. When an unfilled finish is produced, the upper rim surface of the neck of the container will not be complete. Obviously, when this condition occurs, the application of a closure to a filled container will not properly seal on the upper rim surface of the container and will result in a "leaker". Therefore, this possible defect in a container should be inspected for and obviously the defective container removed from the production line. This inspection, preferably, takes place in the glass container plant where the bottles are made. It may be desirable to inspect return bottles which come back to a bottling plant with the neck of the bottle chipped such that it would also produce an unsealed container when the closure is applied. It can be understood that a chipped finish would be somewhat analogous to an unfilled finish in a newly formed bottle. Thus a gauge for inspecting glass containers for "unfilled finishes" may also be used to inspect bottles that have been returned for refilling.
Containers in the past have been examined for unfilled finishes by optical means as shown in U.S. Pat. No. 3,176,842. "Unfilled finishes" are somewhat similar to the defect termed "line-over-finish" which is produced during the manufacture of containers. The "line-over-finish" was caused usually by a blister or bubble, entrapped within the molten glass, being forced into the neck area during "settle blow" and resulting in the blister opening out at the surface of the neck as an elongated void. These defects are more fully described in the above-referred-to patent and also have been inspected by mechanical feeler-type gauges, an example of which is shown in U.S. Pat. No. 3,414,127.